A communication system which performs information communication by wireless or wire is constituted by various radio frequency components such as an amplifier, a mixer, and a filter. As a method to connect these components, there exist various methods for connecting by a coaxial line or a waveguide, or by a planar circuit such as a strip line, a microstrip line, etc.
Since a circumference of a waveguide is enclosed with metals, the waveguide does not have a radiation loss, and has a small insertion loss. Accordingly, the waveguide is a fundamental transmission line frequently used for a radio frequency transmission. The waveguide includes a pipe through which a radio wave transmits, and a flange used for connecting each waveguide circuit. The pipe and the flange are made of metals such as copper, brass, etc. However, since the waveguide employs a metal, the waveguide tends to be heavy to handle, and have a low electrical resistance. The waveguide also has a high heat conductivity of a metal to allow heat to easily move therein. For this reason, there has been a problem that a temperature control for a connection circuit becomes difficult.
In order to solve the problem, waveguides which are designed for a weight saving, or high heat insulation are disclosed. It is disclosed that a pipe and flange portions of a waveguide are molded using a synthetic resin with low heat conductivity, and the surface thereof is plated (JP-A H7-326910 (KOKAI)). It is also disclosed that a waveguide is cooled using cooling fluid around the waveguide (JP-A H4-213902 (KOKAI)). It is further disclosed that a slit is introduced into a portion of a waveguide to lengthen a thermal line length without changing a length of electricity for the waveguide, thus acquiring a heat insulating effect (JP-A H2-311001 (KOKAI)).
However, in any of the above-mentioned waveguides, the metal portions thereof are connected with each other, thereby causing a thermal release. It is tentatively possible to acquire a heat insulation effect by using a metal with low heat conductivity also for other transmission lines, such as a coaxial line, a microstrip line, etc. However, such a low heat conductivity metal has a high electrical resistance, thereby making it difficult to acquire a heat insulating transmission line with a low loss.
A system which operates at low temperatures using a refrigerator, etc. is cooled by housing the system in a vacuum insulating chamber. It is, however, necessary to connect the system and an external circuit for signal communication. A method for connecting the system and an external circuit is disclosed (JP 3466509). The method employs connectors to be fixed to the chamber. The connectors are capable of contacting electrically between the system and the external circuit while maintaining the chamber as a vacuum. However, the method gives rise to heat transfer into the inside of the chamber, because metal parts of the connectors are connected to the inside thereof.
A structure to maintain airtightness of a waveguide employing a dielectric material with a small radio-frequency resistance such as a ceramics, etc. and control a radio-frequency wave reflection due to the dielectric materials is disclosed (JP-A 2007-234343 (KOKAI)). A waveguide having an air gap provided to a choke flange thereof to increase a margin for dimension error of the flange is disclosed (USPA 200800001686).